Agitation of liquid such as oil reserved in a tank is considered necessary for preventing sedimentation of sludge and for maintaining uniformity of blending and/or the temperature of the liquid. To such end, it is customary to use one or more tank mixers in the liquid reserving tank. Such tank mixer conventionally comprises an impeller to be disposed within the tank, a horizontal shaft extending through the wall of the tank and adapted to mount the impeller at the tip thereof, a motor for rotating the shaft and a reduction gear disposed between the motor and the shaft. In a large oil tank, the diameter of which might be several scores of meters, three tank mixers might be installed. In such case, if the tank mixers are installed stationarily, the flow of the liquid induced by the rotation of the impellers is maintained steadily in its direction and some part of the liquid contained in that tank may not be subjected to agitation. Generally, in order to avoid creation of such unagitated portion, the direction of each impeller axis is changed by pivoting each of the mixers as a whole. Such pivotal movement of the tank mixers is usually not effected so frequently, say once a week; however, such pivotal movement of the tank mixers requires special sealing means at the place where the shafts extend through the wall of the tank. For such purpose, a ball shaped member has been affixed to a hollow sleeve inside of which the impeller shaft is rotatably mounted and the ball shaped member together with the hollow tube and the tank mixer are mounted on the wall by a hinge means so as to allow pivotal movement of the tank mixer. When a sealing means is disposed so as to seal the surface of the ball shaped member, this sealing means enables to seal the portion of the wall where the hollow tube extends through the wall of the tank while permitting the pivotal movement of the tank mixer provided that the center of the ball shaped member is on the pivoting axis.
The sealing means described above conventionally uses an annular gland packing surrounding the surface of the ball at the peripheral portion thereof coinciding with the section passing the center of the ball member or at one side of that section member remote from the wall of the tank and this gland packing has been tightened in place by a packing support or backing member.
However, with such conventional sealing means as explained above it is rather difficult to obtain good sealing effect due to the spherical shape of the ball member and the seal often leaks. If the liquid contained in the tank is fresh water, the leakage might not cause serious problem; however, if the liquid is oil, the leakage results in not only an economic loss but also may cause serious problems such as a fire or environmental pollution.
Also, to replace the conventional gland packing it is necessary to first empty the tank. Therefore, it has been long desired to have an effective seal at the wall of the tank without the drawbacks of the prior art.